Regular Expressions in C

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Hi, sorry for the long silence.

I am currently working on a programming project involving a database where I need to check if user input matches a given date format (yyyy-mm-dd).

As simple as it sounds it took me almost an hour to get the regular expression formated until the system finally did what it is supposed to do! So I decided to write it down in case someone has the same issue.

First of all, there is regular expression functionality in C you can use on any POSIX system out of the box.

But what about the syntax? I wanted to use this simple RegEx for the dates:

#define REGEX_DATE „20[0-9][0-9]-(0[1-9]|1[012])-(0[1-9]|[12][0-9]|3[01])“

This does not work – always no match although the RegEx itself compiles.

After some research I found this document. So it seems you have to backslash-escape every control character (like ‚(‚, ‚|‘ but not ‚[‚ or ‚]‘).

So it will be two backslashes until everything works (one backslash makes the compiler think we want to insert some kind of special character).

#define REGEX_DATE „20[0-9][0-9]-\\(0[1-9]\\|1[012]\\)-\\(0[1-9]\\|[12][0-9]\\|3[01]\\)“

And finally it works 🙂

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